1. Field of the Invention
The present invention relates to a power supply control circuit, electronic device, and printing apparatus and, more specifically, to detection of a failure in a power supply circuit having a DC/DC converter.
2. Description of the Related Art
In general, OA equipment having a mechanical part, such as a copying machine or printer, requires at least two types of power supplies having different voltages: a power supply (power supply for generating logic voltages of, e.g., +5 V and +3.3 V) for a logic circuit system for controlling the equipment and a power supply (power supply for generating driving voltages of, e.g., +24 V and +20 V) for a mechanism driving system.
Of the two types of power supplies, the power supply of the driving system must ensure safety for a serviceman and user in maintenance of a driving system component, and save power when the equipment stands by. To meet these requirements, the power supply voltage of the driving system needs a switch for switching the connection state, and must rise and fall in accordance with a specified sequence.
Driving of the equipment upon generation of a state such as an overvoltage in the power supply may deteriorate a driven portion or cause a failure. Thus, driving of the equipment must be avoided upon generation of an overvoltage in the power supply.
The above-mentioned overvoltage is a typical failure of the power supply output. To avoid such an anomaly, a conventional power supply control system suppresses, to a minimum time, a state in which the power supply sequence reverses upon generation of an anomalous output voltage (low voltage or overvoltage) from the power supply unit, or decreases the generation frequency of this state. This measure can avoid malfunction of a logic circuit for driving a load, or prevent deterioration and failure of the load.
For example, Japanese Patent Laid-Open No. 5-204496 discloses a method of turning off the power supply in correspondence with a power supply anomaly signal transmitted from a power supply unit in an arrangement having a plurality of power supply units. According to the method described in Japanese Patent Laid-Open No. 5-204496, at the same time as detection of a power supply anomaly signal, a power supply OFF signal is transmitted to a power supply unit which generated a power supply anomaly and a power supply unit equal to or higher than the anomalous power supply unit in output voltage. A power supply OFF signal is transmitted in accordance with a predetermined power-off sequence to a power supply unit lower in output voltage than the power supply unit which generated the power supply anomaly.
Japanese Patent Laid-Open No. 2000-188829 discloses a method of, when an anomaly occurs in the power supply sequence of a power supply unit, specifying a doubtful power supply unit and clearing up the cause of an operation error by the anomaly of the power supply sequence. According to the method described in Japanese Patent Laid-Open No. 2000-188829, the output power supply voltage of each power supply unit is compared with a specified value at the timing of rising of a power supply output. A doubtful power supply unit is specified on the basis of a logic signal representing the comparison result.
Both the methods described in Japanese Patent Laid-Open Nos. 5-204496 and 2000-188829 detect an anomaly in a power supply sequence when the voltage rises, but do not detect any anomaly in a power supply sequence when the voltage falls.
As for a device requiring periodic exchange of expendables and periodic maintenance, like OA equipment, it is also important to ensure safety in maintenance in addition to preventing deterioration and failure of the load. That is, it is necessary to ensure safety for a serviceman and user when performing maintenance or exchanging expendables. To meet this demand, the power supply voltage must reliably fall to GND level in a falling (power supply-off) sequence, and a generated failure must be detected.